The present invention relates to a method of producing mixtures of cyclic acrolein glycerol acetals. More particulary, the present invention relates to a method for producing mixtures of cis-2-vinyl-4-hydroxymethyl-1,3-dioxolane, trans-2-vinyl-4-hydroxymethyl-1,3-dioxolane, cis-2-vinyl-5-hydroxy-1,3-dioxane and trans-2-vinyl-5-hydroxy-1,3-dioxane. The method is based on the acid-catalyzed reaction of acrolein with glycerol and is especially suited for the continuous production of mixtures of cyclic acrolein glycerol acetals.
The production of cyclic acetals of acrolein with glycerol by means of reacting acrolein and glycerol in the presence of a solid acid catalyst is known; see U.S. Pat. No. 3,014,924 and J. Org. Chem. (1960), pp. 319-324. Highly porous carrier materials coated with mineral acids such as silica, silica gel, and silicoaluminates serve as catalyst. The reaction takes place at temperatures between 50.degree. and 150.degree. C., especially 100.degree. and 125.degree. C., during which the reaction water is distilled of in an azeotropic manner by means of an organic solvent such as benzene, toluene, chloroform or cyclohexane. A disadvantage of this method is the low space-time yield, which can be traced, among other reasons, to the fact that the main amount of the glycerol is at first present as a separate phase. The high reaction temperature required when using a small amount of catalyst entails the danger, in conjunction with the long reaction time, of the formation of byproducts. An elevated amount of catalyst does accelerate the acetal formation but at the same time byproducts are formed in an amount which can not be tolerated.
As regards a continuous process, the use of acrolein as the azeotrope entrainer is advised against as this favors the formation of byproducts. Finally, the use of a heterogeneous organic solvent increases the engineering expense significantly and reduces the economy. The repetition of the method of U.S. Pat. No. 3,014,924 on an industrial scale, as shown herein below, corroborates the unsatisfactory space-time yield and makes it clear that a continuous method can not be operated in an economic manner on this basis.
It is also known that acrolein can be reacted under homogeneous catalysis with a 1,3-diol using mineral acids or sulfonic acids as catalyst (U.S. Pat. No. 4,108,869). The diol is fed into an extraction column in an excess from above and contains the acid. Acrolein is fed into the middle, which acrolein reacts with the downward-flowing diol. A solvent, such as hexane, is fed in from below in countercurrent flow which solvent mixes poorly with the diol. The hexane phase and the diol-water phase are worked up by distillation. This method can not be transferred to the production of acrolein glycerol acetals because acrolein and glycerol do not mix with one another and therefore an insufficient reaction takes place in the extraction column.
A strongly acidic cation exchanger as catalyst for the reaction of 2-methyl-1,3-propanediol with acrolein is suggested in a comparative example of U.S. Pat. No. 4,108,869. The reaction mixture, which is homogeneous at first, separates into an acetal phase and an aqueous phase. Due to the immiscibility of acrolein with glycerol, the previously described method can not be used for an economical production of the desired cyclic acrolein glycerol acetal mixtures.